Environ Eng Res > Volume 26(2); 2021 > Article
Ma, Yuan, Zhang, Qu, and Huang: Inhibiting oxidation and enhancing absorption characteristics of sodium sulfite for SO2 removal from the non-ferrous smelting flue gas

### Abstract

In this work, we investigated the absorption characteristics of SO2 and the effect of inhibitors on the desulfurization performances of Na2SO3. The results showed that the NO2 had a competitive effect with SO2 on SO32− which resulted in a significant decrease in the absorption capacity of SO2. O2 in the flue gas could decrease the absorption capacity of SO2 due to the oxidation of Na2SO3. Besides, Na2S2O3 had more excellent inhibiting effect on the oxidation of SO32−; the inhibition mechanism is understood on the basis of the free radical chain reaction, whereby S2O32− combined with the sulfite free radical to form an inert substance, thus, quenching the reaction of free radical with the dissolved oxygen and invariably inhibiting the oxidation of SO32−. Furthermore, the intrinsic and the apparent oxidation kinetics of Na2SO3 oxidation process with Na2S2O3 were investigated to explain the relationships between consumption rates of SO32− and the absorption capacities of SO2 under different components in flue gas and absorption solution.

### 1. Introduction

The northern area of China has witnessed incessant haze pollution in recent years. This predominant in cities with high industrial activities, and occurs mainly during the winter or spring season [12]. The major component of haze is the particulate matter (PM). PM2.5 (particulate matter with diameters less than 2.5 μm) have more concentrations and it is more harmful to human health, this is mainly attributed to its small size which is an influential factor to visibility since mass extinction efficiency of PM2.5 is 7 times of larger particles. Moreover, the deterioration of air quality and haze episodes illustrates that PM2.5 plays a dominant role in the formation of smog [34]. The visible light absorption and scattering were affected by the gaseous pollutants in the air and the chemical compositions of PM2.5 [5]. Sulfur dioxide (SO2) is the primary haze pollutant and contributes significantly to the nucleation, toxicity, and complexity of PM2.5 [68]; hence controlling its emission is of paramount importance. Furthermore, it has been reported that sulfate, nitrate, and ammonium (SNA) which are referred to as secondary inorganic aerosols contribute to approximately 27 % of the PM2.5.
To ensure that the SO2 and NOX emissions from flue gases are reduced to the barest minimum, the desulfurization and denitrification technologies in coal-fired power plants and the industrial boilers have been developed for years in compliance with the stringent emission standard [913]. However, the SO2 emission from the non-ferrous metal smelting flue gas has received little attention in the past, hence, the control of SO2 emission has not been optimized [14].
In a typical smelting process, a high concentration of SO2 (>4%) can be directly converted to sulfuric acid by double-conversion double-absorption (DCDA) process downstream of the dust removal devices and dynamic wave scrubbing units [15]. While for the SO2 concentration in the range of 2 – 4% in flue gas, single-conversion single-absorption (SCSA) process is employed to produce sulfuric acid and the SO2 in the exhaust gas can be absorbed by sodium hydroxide. However, when SO2 in flue gas is lower than 2%, it cannot be used for the conversion of sulfuric acid due to its low concentration. Wet scrubbing technology using sodium hydroxide is adopted for desulfurization in these smelting factories. The concentration of SO2 was still too high to be controlled, unlike the concentration of gases from coal-fired flue gas.
The use of sodium hydroxide, sodium citrate process, organic amine process, ionic liquid process, magnesium process, and sodium sulfite process for the removal of SO2 from characteristics flue gas have been investigated and implemented, but these processes are not economical. Moreover, they lead to heavy metal pollution and hazardous waste generation [1619]. Sodium sulfite process (Wellman-Lord Process) is one of the most efficient technique for the desulfurization and recovery of SO2, due to its relatively high efficiency and low energy consumption [20]. However, sodium sulfite process is offset by the reduction in the desulfurization efficiency, which is the consequence of Na2SO3 oxidation in the absorption solution. The following equations show the main absorption and oxidation processes of Na2SO3 [21]:
##### (1)
$Na2SO3+SO2+H2O=2NaHSO3$
##### (2)
$2Na2SO3+O2=2Na2SO4$
##### (3)
$6NaHSO3=2Na2SO4+Na2S2O3+3H2O+SO2$
It was reported that the oxidation of sulfite was a free radical reaction. Sipos et al. [22] selected phenol as the appropriate inhibitor. However, phenol was not acceptable for real applications because it is also an environmental pollutant. Cui et al. reported that the hydroxyl functional group in TP (Tea Polyphenols) makes the ring become active and reductive, hence TP can be effectively used as an additive. The chain reaction of sulfite oxidation was broken and inhibited [23]. Wang et al. [24] studied the effect of ethanol on the oxidation of Na2SO3 and reported that ethanol inhibited the oxidation of sulfite. The mechanism of inhibition of sodium sulfite oxidation and the inhibition kinetics of sulfite oxidation by ethanol was proposed. However, previous studies primarily focused on selecting inhibitors of sulfite oxidation and investigating the inhibition mechanism, but they are not applicable in non-ferrous smelting industries. The selection and inhibition mechanism of complex flue gas with high concentrations of SO2, NOX, and O2 is not fully understood, hence, it requires further studies. Besides, the relationships between desulfurization performances and the sodium sulfite added inhibitors are still ambiguous, and the effects of complex components in smelting flue gas on the inhibitors are not clear.
In this study, we investigated the absorption characteristics of SO2 along with the intrinsic and apparent oxidation kinetics of SO32− in the cyclic sodium sulfite absorption system. The related kinetic parameters and mechanism of inhibition oxidation were obtained by investigating the relationship between the consumption rate of SO32− and the absorption time of SO2 under different components in flue gas and absorption solution with Na2S2O3 inhibitor. The results will provide a theoretical basis for SO2 emission control and resource recovery from non-ferrous metal smelting flue gas.

### 2.1. Experimental Apparatus

A schematic diagram of the experimental apparatus was shown in Fig. S1 (as shown in supplementary material), and it is made up of the simulated flue gas distribution system, the absorption reaction system, the online monitoring system, and the exhaust gas treatment system. The simulated flue gas was supplied by the cylinders at a flow rate of 1.0 L/min. The components in flue gas were 5000 mg/m3 SO2, 600 mg/m3 NO2, 0 – 8 % O2 and N2 of the carrier gas. All the gases were controlled by mass flow meters. The SO2 absorption experiments were carried out in a three-port bubbling reactor with 50 mL of 2 – 4% Na2SO3 solution. The temperatures of flue gas and absorption solution were 323K and 293 K, respectively. The flue gas from the outlet was continuously absorbed by 5% NaOH and 5% KMnO4 solution so that the exhaust gas was purified and discharged.
The concentration of SO2 and NOX in the inlet and outlet of the reactor was monitored by a flue gas analyzer (Testo-350, Germany).

### 2.2. Materials

The main chemicals and consumables used in this study are stated below: sodium sulfite (99%), sodium thiosulfate (99%), sodium sulfate (98%), sodium nitrite (99%), anhydrous ethanol and triethanolamine from Sigma-Aldrich Co, Ltd. The SO2 (5%), O2 (99.9%), NO2 (1%) and N2 (99.9%) were obtained from Henan Yuanzheng Gas Co., Ltd and stored in cylinders.

### 2.3. Methods

The removal efficiencies of SO2 were calculated by the following Eq. (4):
##### (4)
$ηSO2=CSO2(i)-CSO2(O)CSO2(i)×100%$
Where ηSO2 is the removal efficiency of SO2, and CSO2(i) and CSO2(O) are the sulfur dioxide inlet concentration and outlet concentration, respectively.
The absorption capacities of SO2 in Na2SO3 solution were calculated by integrating the absorption curve as shown in Eq. (5).
##### (5)
$τ=1V∫t2t1(C(i)-C(o))×f×dt$
where τ is the absorption capacity of SO2, and C(i) and C(o) are the concentrations of the SO2 inlet and outlet, respectively. V is the volume of Na2SO3 solution. t1 and t2 are the starting and ending time of absorption, respectively. f is the flue gas flow.
The consumption of Na2SO3 was deduced from the Eq. (1) so that the utilization ratio of Na2SO3 was obtained by the following Eq. (6):
##### (6)
$η1Na2SO3=mNa2SO3(c)mNa2SO3(t)×100%$
where η1Na2SO3 is the utilization ratio of sodium sulfite, and mNa2SO3(t) and mNa2SO3(c) are the total amount of sodium sulfite and the amount of sodium sulfite consumed by absorbing SO2, respectively.
The intrinsic oxidation kinetics parameters and inhibition mechanism of Na2SO3 were obtained by the following methods. Na2SO3 reacted with dissolved oxygen according to the Eq. (7). The dissolved oxygen meter records the concentration of dissolved oxygen in solution. Compared with the initial concentration of SO32− (C2) and dissolved oxygen (C3), the initial concentration of the inhibitor (C1) is negligible. At any time, the consumed dissolved oxygen is marked to C3i, and the concentration of oxidized SO32− is 2 (C3i), and then plotted as 2 (C3i)-t. The slope of the obtained straight line is the intrinsic oxidation rate of SO32−.
##### (7)
$SO32-+1/2O2→SO42-$
When the temperature is constant, the reaction rate of SO32− at different initial Na2S2O3 concentration (Ci) is determined, and then plotting with Ln(r)-Ln(ci), the slope of the obtained straight line is the reaction order of the inhibitor. Similarly, by measuring the reaction rate of SO32− at different initial Na2SO3 concentrations (Cj), and then plotting it with Ln (r) -Ln (Cj), the slope of the obtained straight line is the reaction order of SO32−.
The apparent oxidation kinetic parameters and inhibition mechanism of Na2SO3 could be obtained by the following methods. The liquid samples taken from the experiment were quantitatively analyzed by Dionex Aquion analytical multi-function ion chromatography (Dionex Aquion, Thermo Scientific, America). The generated SO42− concentration (C4i) at different absorption times was measured, and then plotted as C4i-t. The slope of the obtained straight line was the SO42− generation rate. Similarly, by measuring the SO42− generation rate at different initial Na2S2O3 concentrations (Ck), and then plotting it with Ln (r) -Ln (Ck), the slope of the obtained straight line is the reaction order of the inhibitor.

### 3.1. Effects of Flue Gas Components on Desulfurization Performance

The effects of the primary gas components on desulfurization performance were investigated and the results are shown in Fig. 1. The total flow rate of the simulated flue gas was 1.0 L/min. The initial SO2 concentration was 5,000 mg/m3, and the Na2SO3 mass concentration was 2%. The results showed that O2 and NO2 in flue gas had a significant effect on the effective desulfurization time, but had a negligible effect on the maximum desulfurization efficiency. Curves of a, b, and d in Fig. 1 revealed that the absorption capacity of SO2 decreased with the presence of O2 in the flue gas. The higher the O2 concentration, the smaller the absorption capacity of SO2. This may be attributed to the consumption of SO32− by O2, as shown in the Eq. (2). When the oxygen concentration increased from 0 to 6%, the absorption capacities of SO2 decreased from 9.96 to 8.69g/L, respectively. Accordingly, the increase in the concentration of SO42− from 0 to 5.86g/L detected in the absorption solution indicated that part of SO32− was directly oxidized to SO42−, thereby reducing its participation in the SO2 absorption reaction.
Denitration in a non-ferrous metal smelting flue gas usually involves the oxidation of NO to NO2 by ozone and the absorption by aqueous solution. As can be seen from the curves of b and c in Fig. 1, when NO2 was introduced into the flue gas, the absorption capacities of SO2 decreased from 8.69 to 3.04g/L, which indicated that NO2 in flue gas is unfavorable for the absorption of SO2. Irrespective of its excellent solubility in water, NO2 still have a poor absorption effect in aqueous solution than in Na2SO3 solution from Fig. S2. The absorption of NO2 by Na2SO3 solution certified that NO2 and SO2 had a competitive effect on SO32−. The consumption of SO32− by NO2 as shown in the Eq. (8)(11) is the main reason for the decrease in the absorption capacity of SO2. After the absorption of pure NO2 by Na2SO3, the SO32− in the absorption solution was converted to SO42−, this observation validates the role of Eq. (8)(11) in the absorption system.
The absorption curves of SO2 with the presence of NO2 were shown in Fig. S3. Compared with curve b in Fig. 1, the absorption capacity of SO2 decreased from 8.69 g/L to 3.04 g/L. Compared with the absorption of NO2 by 2% Na2SO3 in Fig. S2, the absorption capacity of NO2 decreased from 0.33 g/L to 0.21 g/L, which proved that NO2 and SO2 had competitive effects on SO32−.In addition, NO2 is absorbed to form NO2, which can react with SO32− to form the intermediate sulfur nitrogen compound, which can be stable at a high S/N ratio and low pH [25, 26]. The reaction equation involved is shown as Eq. (8)(11):
##### (8)
$SO32-+2NO2+H2O→2NO2-+SO42-+2H+$
##### (9)
$HSO3-+2NO2+H2O→2NO2-+SO42-+3H+$
##### (10)
$SO32-+2NO2-+4H+→2NO+2H2O+H2SO4$
##### (11)
$NO2-+H++HSO3-→ONSO3-+H2$

### 3.2. Selection of Oxidation Inhibitor for S(IV)

To inhibit the oxidation of Na2SO3, the effects of different inhibitors on the desulfurization performances of Na2SO3 were investigated. The results are shown in Fig. 2. When 34.28 mmol/L of ethanol was added into the absorption solution, the absorption capacity of SO2 increased from 8.47 to 8.49 g/L, and the utilization rate of Na2SO3 increased from 83.37% to 83.57%, indicating that ethanol had a slight inhibiting effect on the oxidation of SO32−. However, increasing the concentration of ethanol in the absorption solution did not promote the inhibiting effect of ethanol on the oxidation of SO32−, this observation is attributed to the expulsion of excessive ethanol from the absorption system by the simulated flue gas. Moreover, this assertion is also validated by the presence of ethanol detected in the tail gas.
When ethylene glycol was added as an inhibitor to the absorption solution, the oxidation of SO32− was also inhibited. The ethylene glycol will act as the •SO3 radical-trapping material and break the chain by reacting with the •SO3 radical. Finally, ethylene glycol may generate aldehyde or carboxylate as a result of induced oxidation of ethylene glycol. Increasing the concentration of ethylene glycol led to an insignificant improvement in its inhibiting effect. It can be seen from Fig. 2 that when Na2S2O3 was used as an inhibitor, the absorption capacity of SO2 increased obviously.
Also, the inhibiting effect of triethanolamine on oxidation of SO32− was investigated. As seen from Fig. 2, the absorption capacities of SO2 increased to 28.45% after the addition of 34.28 mmol/L of triethanolamine into the absorption solution, which primarily indicated that triethanolamine can promote SO2 absorption in this system. However, the triethanolamine solution presents alkaline and produces ammonium ions, which may be one of the reasons for promoting the absorption capacity of SO2. Therefore, further exploration was preceded and the results were shown in Fig. 3. In comparison with 2% Na2SO3, the pure 0.2% triethanolamine absorbed more SO2. Therefore, the addition of triethanolamine mainly promotes the absorption capacity of SO2 Eq. (12), and to some extent, inhibits the oxidation of SO32−.
##### (12)
By comparison, the inhibiting effect of Na2S2O3 on oxidation of SO32− was more excellent and matched with the Na2SO3 desulfurization system. Then, the effects of Na2S2O3 as an inhibitor on the desulfurization of the Na2SO3 system were investigated.

### 3.3. Effects of Na2S2O3 Inhibitor on Desulfurization Performance

Increasing Na2SO3 concentration can improve desulfurization efficiency and increase the absorption capacity of SO2. However, it also increased the oxidation rate of SO32−, resulting in a lower utilization rate of Na2SO3. The effects of Na2SO3 concentration on desulfurization were investigated at a constant Na2S2O3 concentration. From Fig. 4(a), in the absence of Na2SO3, the absorption capacities of SO2 by 2%, 3% and 4% Na2SO3 was 8.83g/L, 11.77 g/L, 15.06 g/L, and the utilization rate of Na2SO3 was 86.88%, 77.22%, and 74.11%, respectively. When Na2S2O3 was added into the absorption solution as an inhibitor, the absorption capacities of SO2 by 2%, 3% and 4% Na2SO3 were increased to 19.48%, 31.52%, and 33.07%, respectively.
In order to clarify the dosage relationship between Na2S2O3 and the oxidation of Na2SO3, the inhibiting effects of Na2S2O3 concentration were investigated at constant Na2SO3 concentration. As seen from Fig. 5, pure Na2S2O3 solution had poor desulfurization efficiency, but the absorption capacity of SO2 was enhanced significantly after adding Na2S2O3 as an inhibitor. The absorption capacity of SO2 increased progressively with the increase in Na2S2O3 concentration and attained a maximum level when the concentration of Na2S2O3 is 240 mmol/L. Therefore, the concentration of Na2S2O3 should be matched with the concentration of Na2SO3 to optimize the dosage of inhibitors.
The oxidation of Na2SO3 was prompted by the oxygen from the flue gas which dissolved into the absorption solution and reacted with SO32− to produce stable SO42−, so the concentration of oxygen in the flue gas was also one of the main factors affecting the SO2 removal. As shown in Fig. 4(b), the increase of oxygen concentration without inhibitor led to the increase in the oxidation rate of SO32− and the decrease in the absorption capacity of SO2. When Na2S2O3 was added as an inhibitor, the increase in oxygen concentration had little influence on the oxidation rate of SO32− and absorption capacity of SO2.
The inhibition mechanism of Na2S2O3 on the oxidation of Na2SO3 may be summarized as follows: the oxidation reaction of SO32− is a free radical chain reaction and the inhibitor mainly inhibits the oxidation of sulfite by preventing the chain reaction. The S2O32− combines with the sulfite free radical to form an inert substance that prevents the sulfite free radical from further reacting with the dissolved oxygen. The reaction mechanisms are shown in Eqs. (13)(16) [27]:
##### (13)
$·SO3-+S2O32-→·S2O3-+SO32-$
##### (14)
$·SO3-+·S2O3→polythionates$
##### (15)
$·S2O3-+·S2O3-→polythionates$
##### (16)
$polythionates+H2O→S2O32-+SO32-+2H+$

### 3.4. The Kinetics of SO32− Oxidation and Its Inhibition

The intrinsic oxidation kinetics and apparent oxidation kinetics of sulfite were studied to properly elucidate the oxidation mechanism of sulfite. The intrinsic oxidation kinetics are shown in Fig. 6 and Fig. 7. It can be seen from Fig. 6, when the concentration of Na2S2O3 was 0.10, 0.15 and 0.2mmol/L, respectively. The initial concentration method was used to obtain the reaction rate of sulfite under different inhibitor concentrations. The sulfite concentration and reaction rate was dimensionless with respect to the initial values. Therefore, as shown in Fig. 6, the reaction order of Na2S2O3 was −2.22. This indicates that Na2S2O3 inhibits sulfite oxidation significantly, and the sulfite oxidation rate decreased significantly with the increase in Na2S2O3concentration.
The reaction order of sodium sulfite under different initial Na2SO3 concentration was shown in Fig. 7. When 0.20 mmol/L of Na2S2O3 was added, and the initial concentration of sodium sulfite was 4, 6, 8, 10 mmol/L, the sulfite concentration and reaction rate was dimensionless with respect to the initial values. Therefore, the reaction order of Na2SO3 as shown in Fig. 7 was 2.393, which indicates that the oxidation rate of sulfite increases with the increase in the initial concentration of Na2SO3.
The apparent oxidation kinetics mechanism was obtained by analyzing the composition of the desulfurization solution. Dionex Aquion analytical multi-function ion chromatography was used to quantitatively analyze the desulfurization solution with or without inhibitor. As can be seen from Fig. 8, the rate of sulfate formation was 53.08 mg/(L·min) in the absence of inhibitor. When 240mmol/L of inhibitor was added, the rate of sulfate formation was 7.28 mg/(L·min), which indicated that Na2S2O3 inhibits sulfite oxidation, thereby reducing the rate of sulfate formation.
The reaction order of sulfate at different initial Na2S2O3 concentration was shown in Fig. S3. On the addition of 80, 160, and 240 mmol/L of Na2S2O3, respectively, the sulfate content was determined by ion chromatography, from which the reaction rate was calculated. Since the sulfate concentration and the reaction rate are dimensionless with respect to the initial value. Therefore, the reaction order of Na2S2O3 as shown in Fig. S3 was −1.35. This result revealed that Na2S2O3 inhibits the rate of sulfate formation significantly, and the rate of sulfate formation decreases significantly with increasing concentration of Na2S2O3.

### 4. Conclusions

In this study, a new cyclic sodium sulfite process was proposed to remove SO2 from a non-ferrous metal smelting flue gas. The effects of different components of flue gas on desulfurization performance were investigated, and the results indicated that the NO2 had a competitive effect with SO2 on SO32− which led to a significant decrease in the absorption capacity of SO2. The O2 in the flue gas contributed to the decrease in the absorption capacity of SO2 due to its oxidative effect on Na2SO3. By comparison, the inhibiting effect of Na2S2O3 on the oxidation of SO32− was more excellent and matched with the Na2SO3 desulfurization system. The inhibition mechanism is understood on the basis of the free radical chain reaction, whereby S2O32− combined with the sulfite free radical to form an inert substance, thus, quenching the reaction of free radical with the dissolved oxygen and invariably inhibiting the oxidation of SO32−.
In addition, the intrinsic oxidation kinetics showed that the reaction order of Na2S2O3 was −2.22, and the reaction order of Na2SO3 was 2.393. On the addition of 240 mmol/L of Na2S2O3 in Na2SO3 solution, the apparent oxidation kinetics gave a sulfate formation rate of 7.28 mg/(L·min). However, in the absence of Na2S2O3 in the Na2SO3 solution, the formation rate of sulfate was 53.08mg/(L·min). The reaction order of Na2S2O3 was −1.35, which indicated that Na2S2O3 has an excellent inhibiting effect on the oxidation of SO32− and the desulfurization performances are improved significantly.

### Acknowledgment

This study was supported by the National Key R&D Program of China (No.2017YFC0210500), Key Scientific Research Project of Colleges in Henan Province-China (No.20A610011), the China Postdoctoral Science Foundation (No.2018M640393). This work was also funded by the Scientific and Technological Project of Henan Province-China (No. 202102310283).

### Notes

Author Contributions

YP.M. (Associate professor) contributed to the conception of the study. DL.Y. (Master student) conducted all the experiments and wrote the manuscript. XJ.Z. (Associate professor) contributed to analysis and manuscript preparation. Z.Q. (Professor) performed the data measurement and analyses. WJ.H. (Associate professor) helped perform the analysis and writing examination.

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##### Fig. 1
Effects of flue gas components on desulfurization performances.
##### Fig. 2
Effect of different additives on desulfurization performances.
##### Fig. 3
Effect of different triethanolamine content on desulfurization performances.
##### Fig. 4
(a) Effect of Na2SO3 Concentration on Desulfurization Performance, (b) Effect of oxygen content on desulfurization performance of Na2SO3.
##### Fig. 5
Effect of Na2S2O3 concentration on desulfurization performance.
##### Fig. 6
Reaction order of Na2S3O3.
##### Fig. 7
Reaction order of sulfite.
##### Fig. 8
Effect of Na2S2O3 on sulfate formation rate.
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